Republicans' chances of gaining enough seats for Senate control begin to fade as more races start leaning toward Democratic candidates.

WASHINGTON — The battle for control of the Senate, which had favored Republicans for much of this election year, has abruptly shifted, with Democrats sharply improving their odds of keeping the majority.

Along with recent missteps by presidential nominee Mitt Romney, the erosion of Republican prospects in the Senate has contributed to a grim feeling among party activists that an election year that once held out the possibility of regaining full control of the government could be souring on them.

Republicans entered the election season with fewer incumbents to defend and many opportunities to pick up seats from Democrats at a time when President Obama's approval rating had dipped to a low point.

They need to gain a net of four seats to wrest away the majority that Democrats have held since 2006 — or three seats if Romney takes the White House, giving his vice president the tie-breaking vote. With nearly two dozen seats held by Democrats up for grabs this November, the routes to a GOP majority were many.

But in races from Virginia to Wisconsin, recent polls have shown a shift in Senate contests in favor of the Democrats, and the weakness of some GOP candidates has limited the party's prospects.

Strategists in both parties say that part of the problem may be that Romney's campaign stumbles are hurting Republican candidates. At the same time, Obama and his fellow Democrats have enjoyed a lift since their national convention, which energized supporters who may not have been paying close attention to the races.

In one of the marquee races of the season, the Democratic candidate in Massachusetts, Elizabeth Warren, took a lead in four recent surveys after a prime-time turn on the stage at the convention. Incumbent Republican Sen. Scott Brown led in one poll. The seat is a must-hold for the Republicans, but Romney, the former governor, is expected to lose the state by a large margin, creating a tough environment for Brown.

Another Republican-held seat in New England became a problem for the party earlier this year when Maine's Olympia J. Snowe announced her retirement. Former Gov. Angus King, an independent, is heavily favored to win. He has not said which party he would side with if elected, but Republicans have campaigned hard against him, believing that he leans toward the Democrats.

And as those Republican-held seats have become unsafe, some pickups the party had been counting on have begun looking less certain.

In Virginia, where the contest was dead-even for months, Democratic candidate Tim Kaine appears to have taken a lead over George Allen in a battle between two former governors. And in Wisconsin, three recent polls have shown Democrat Tammy Baldwin moving into at least a tie with former Gov. Tommy Thompson. Both seats are open because incumbent Democrats are retiring.

Some of the recent Democratic gains could fade, analysts note.

"You've had perhaps the worst month of the campaign for Romney," said Bob Holsworth, a longtime political analyst in Virginia. "And Obama had a good month, and these polls reflect that."

As in the presidential race, many Senate campaigns are being fought among a slim slice of undecided voters, and the quirks of each matchup can play a decisive role.

"Candidates matter," said Jennifer Duffy, who analyzes races for the nonpartisan Cook Political Report.

In Missouri, for example, after Republican Todd Akin made his comments on "legitimate rape" last month, key party leaders essentially walked away from the race. Some conservatives such as Newt Gingrich have rallied to Akin, but polls show the GOP may lose perhaps its best opportunity to defeat a Democratic incumbent, first-term Sen. Claire McCaskill.

Even in heavily Republican North Dakota, Democrat Heidi Heitkamp, a former state attorney general, is putting up a strong fight against Rep. Rick Berg for a longtime Democratic seat that Republican strategists had counted on capturing.

Neither party is expected to sweep the election, as happened in 2006, when Democrats took control of Congress during the backlash against George W. Bush and the Iraq war, or in 2010, when Republicans rode tea party enthusiasm to the House majority.

Several Democratic senators up for reelection initially appeared vulnerable because they were first elected on that earlier wave six years ago. But Republican hopes of defeating those incumbents have cooled. In Ohio, for example, the challenger to Democratic Sen. Sherrod Brown has struggled.

In the swing state of Florida, Republicans have not produced a strong challenge to Democratic Sen. Bill Nelson.

"The perception is we have weak candidates all around," said one GOP strategist, granted anonymity to speak frankly about the situation. Still, he noted, "We're far, far from throwing in the towel."